Overweight and obese women -- defined as having a BMI (body mass index) higher than 25 -- have a higher risk of being diagnosed with breast cancer compared to women who maintain a healthy weight, especially after menopause. Being overweight also can increase the risk of breast cancer coming back (recurrence) in women who’ve been diagnosed with the disease.

This higher risk is partially because fat cells make estrogen; extra fat cells mean more estrogen in the body, and estrogen can make hormone-receptor-positive breast cancers develop and grow. Scientists also have recently found that extra fat cells can trigger long-term, low-grade inflammation in the body. Chronic inflammation has been linked to a higher risk of breast cancer recurrence; the proteins secreted by the immune system seem to stimulate breast cancer cells to grow, especially estrogen-receptor-positive breast cancer in postmenopausal women.

Many people gain weight during and after breast cancer treatment, especially if they’re treated with chemotherapy, steroids, and/or hormonal therapy. If breast cancer treatment puts you into menopause, you’re more likely to gain weight.

There are dozens of studies that have shown that losing weight, exercising more, and eating a healthy diet improve the physical and mental well-being of people who’ve been diagnosed with breast cancer. People who make these changes also see positive changes in biomarkers (measurable characteristics in the body) linked to breast cancer risk and outcomes.

But as many of us know, losing weight can be hard and frustrating.

A study found that with targeted support and encouragement, overweight and obese women who had been diagnosed with breast cancer were able to make lifestyle changes that helped them lose about 6% of their body weight after a year.

In the study, called the ENERGY trial, 692 overweight or obese women who had been treated for early-stage breast cancer about 2 years earlier were randomly assigned to one of two groups:

The lifestyle intervention group: These women attended motivational group meetings focusing on increasing physical activity and decreasing the number of calories they ate. They also received telephone counseling and newsletters tailored to their specific weight loss needs.

The control group: These women received information about lifestyle changes they could make to lose weight, but didn’t go to any group meetings, didn’t receive telephone counseling, and didn’t receive tailored newsletters.

The study lasted for 2 years and took place at four locations in the United States: Denver, San Diego, St. Louis, and Birmingham, Ala.

During the first year, the women in the intervention group attended the motivational group meetings every week at first, and then once a month. In the second year, the women received the tailored newsletters and telephone counseling to encourage them to continue their lifestyle changes.

The goal of the study was for the women in the intervention group to lose 7% of their body weight and maintain the weight loss for 2 years. The researchers measured the women’s weight and blood pressure five times during the study: before it started, 6 months later, 12 months later, 18 months later, and 24 months later.

After the study’s first year, the researchers found that while both groups of women lost weight, the women in the intervention group lost much more:

women in the intervention group lost 6% of their body weight

women in the control group lost 1.5% of their body weight

After 2 years, when the study ended:

average weight loss in the intervention group was 3.7% of body weight

average weight loss in the control group was 1.3% of body weight

The women in the intervention group also had much higher physical activity levels and much lower blood pressure levels than women in the control group after the study ended.

The weight loss intervention strategies seemed to be more effective among women who were older than 55 compared to women who were younger.

"This is a significant finding because it shows that women are able to lose weight after breast cancer treatment, and this may lead to reduced risk of recurrence," said Tim Byers, M.D., M.P.H., associate director for cancer prevention and control at the Colorado University Cancer Center and one of the study’s authors. "Although we have not yet proven that weight loss reduces the chance of recurrence, I think these findings show that we should now move forward with a larger study. If we can prove that intentional weight loss reduces breast cancer recurrence, I believe weight control will become a routine part of oncology care.

“We don’t need to focus on getting to the ‘ideal’ body type, whatever that may be,” he continued. “If we can prove that modest weight loss improves the chance of better outcomes after breast cancer, we need to encourage and support that.”

Losing weight can be hard after breast cancer treatment, but as this study shows, it can be done with careful changes to your diet and regular exercise. The first thing to do is to talk to your doctor about a healthy weight for you based on your age, height, body type, and activity level. Then ask your doctor about a safe and sensible plan to lose weight specifically designed for you and your needs.

Once you have the OK from your doctor and a weight goal, you can create a healthy eating plan that meets your nutritional needs. You may want to talk to a registered dietitian about how to create a healthy eating plan that’s tailored to your specific needs and likes.

Some women say that it helps to think of eating well and exercising as important parts of their treatment plans. Remember to be nice to yourself; don’t punish yourself.

In the Breastcancer.org Nutrition section, the Eating to Lose Weight After Treatment pages can help you assess your weight and create a healthy eating plan. And the Breastcancer.org Exercise section can help you find a trainer and learn how to stick to an exercise routine.